If a week is a long time in Politics, what’s a decade in Telecoms? According to BT that’s how long it will take the UK to transition from the current PSTN/ISDN service to a unified, full-fibre network. Any business currently using ISDN lines will be obliged to move away from the platform.  Why is the change occurring, what options are available, and is it best to switch now?


isdn-timetableIn 2015, BT announced that it would be phasing out its ageing copper-based ISDN lines and PSTN service, and replacing them with a full fibre network. Related services like Whole Line Rental (WLR), Asymmetrical Digital Subscriber Lines (ADSL) and Fibre to the Cabinet Broadband (FTTC) would also be affected. In short the biggest shake-up for Telecoms since the 1980s.


A timetable was drawn up so organisations and telecoms providers could plan for change. The most important one is that in December 2025, traditional telephony, fixed lines and services in the main Public Switch Telephone Network will be turned off altogether.

2015 – BT announces it will be phasing out ISDN

2017 – the year 2025 is confirmed as the date for the ISDN switch-off

2018 – Openreach’s Fibre First strategy announced. BT also reports that 2m UK businesses still needed to change to SIP or cloud telephony

2020 – Halfway reminder that Wholesale Line Rental (WLR) to be phased out

2023 – BT will stop selling any PSTN/ISDN/WLR services

2025 – End of Life: PSTN and ISDN withdrawn

The numbers illustrate the scale of the task. BT has to replace 33.2 million UK landlines with a new technology fit for the next 30 years.


Most of us probably don’t know what PSTN and ISDN stand for, even though we take them for granted every day. They represent the ‘traditional telephony’ circuits that most of us still currently use to communicate over the phone.

PSTN (Public Switched Telephone Network) – this technology allows analogue voice data to travel from one point to another through circuit-switched, copper-based phone lines.

ISDN (Integrated Services Digital Network) – allows voice and data services to be carried over digital lines. At the time this facilitated faster internet connections, multiple calls over the same line, video conferencing and DDIs (direct dial-in numbers).


the-end-of-isdn-and pstnBoth PSTN and ISDN are legacy technologies reaching the end of their life, eclipsed now by different 21st century alternatives. Modern technology has far outstripped the capability of traditional telephony. The PSTN now supports an almost entirely digital network platform. This never existed when it was first deployed in early telephone exchanges from the 1920s. Improved quality and reliability of broadband is ringing the changes. Traditional fixed line, and fixed analogue services are being replaced, and in time will cease to exist.

Managing multiple legacy networks has become expensive for Openreach, BT’s standalone digital network division. The priority for BT now is for all of these services to be converged into a single, ‘fibre first’ IP network. Cheaper to maintain, easier to run, this will create better connectivity options for businesses and consumers.


This change is not just happening in the UK. Britain will join other European countries, all at different stages of their ISDN switch-off. Macedonia was the first to switch off its ISDN lines (in 2014) and other countries like France, Switzerland and Belgium will follow.

Overlaying a new phone network onto the existing one, without any loss of connectivity, is no small undertaking. It is hoped that BT can learn from the experience of Deutsche Telecom AG. After overcoming many hurdles it is now on course to shutting down all of Germany’s landline connections. Highlighting the challenge, Dirk Woessner, CEO of Telekom Deutschland said “it was like changing tyres at full speed”.

Here in the UK, towns like Salisbury and Mildenhall are already taking part in full fibre trials.  BT has also created Single Order Transitional Access Products (SOTAPS), an apparent temporary solution.These will keep parts of the copper network open for internet connections, even if it is no longer used for Voice itself.  It is likely that these will be maintained for as long as it takes BT to get FTTP (Fibre To The Premises) into our businesses and homes.


BT’s future investment will be in a fibre-based IP network. Fibre presents businesses with two main options for replacing their existing ISDN phone lines:

SIP Trunks are virtual telephone lines over your internet connection, thereby eliminating the need for a physical phone line altogether. With SIP Trunks, organisations are no longer limited by location. As long as there is a connection to the Internet they can be set up anywhere, making them ideal for remote working from different locations. They can also be scaled up or down, enabling businesses to exert greater control over call volumes and hence make cost savings.

VoIP Phone Systems allow calls to be made over an Internet connection, either with an on-premise or hosted cloud solution. VoIP systems are easily accessible via multiple devices, enabling employees to connect and communicate from desk phones, mobiles, tablets and computers. Simple to set up, cloud-based phone systems have shown themselves to be flexible, economical solutions for thousands of companies.


If your business is still using ISDN lines, and you want to ensure you have everything in place for a smooth transition, we recommend the following:

  • Avoid a last-minute panic, and possible installation premiums, by planning for change now.
  • Check your current phone contract. Be wary about signing up to any further ISDN/PSTN obligation.
  • Evaluate and compare SIP Trunk, Hosted and VoIP options.
  • Find out if your existing Phone System already supports VoIP technology – or ask Clarion to confirm that for you.
  • Review any other business applications that run off broadband


Clarion can certainly help your business to decide which phone system is best for your organisation. Remember too that the BT change will also affect other services that depend on a broadband connection. We can advise how to modify your business alarm system, CCTV security cameras, card payment terminals and fax machines to make sure they run smoothly. Call us on 0333 222 6633 or email us for help.

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